The improvement of current anti-viral vaccines and the development of novel vaccines depends on increasing our understanding of viral attachment and fusion glycoproteins. Critical insight into understanding the antigenic structure of glycoproteins is provided by studying their interaction with monoclonal antibodies (mAbs). For a number of years we have studied the influenza virus hemagglutinin (HA) glycoprotein. This protein serves as a model for other proteins with similar functions (e.g. HIV gp160), and moreover, is important practically in its own right, as influenza still is a major cause of morbidity and mortality nationally, and internationally. Like many viral glycoproteins the HA is a homo-oligomer, consisting of three identical monomeric subunits. In the past year we continued to investigate the site of trimerization of newly synthesized HA. Our previously published findings suggested that HA trimerization occurs only after monomers are exported from the ER. This conclusion was based largely on localization studies performed with a standard immunofluorescence microscope. In the past year we have increased the resolution of detection by using a laser confocal scanning microscope to detect fluor-tagged antibodies, and an electron microscope to detect colloidal gold tagged antibodies.